1. Field of the Invention
The present invention generally relates to a temperature sensor, and more particularly, to a temperature sensor for detecting the operating temperature of, for example, an LIS or other semiconductor devices, which is capable of stable operation in a high temperature range, while realizing at least one of a highly sensitive operating mode and a low-voltage operating mode.
2. Description of the Related Art
If an abnormally large quantity of electric current flows through a semiconductor integrated circuit, or if the temperature of the semiconductor integrated circuit rises too high due to the environmental change, the semiconductor integrated circuit will be destroyed. To prevent the destruction, the operation of the semiconductor integrated circuit has to be stopped before the detected temperature reaches the critical temperature. To this end, a temperature sensor or a temperature protection circuit is generally incorporated in the semiconductor integrated circuit to prevent the circuit from being damaged. Such a temperature sensor includes a PTAT voltage generator that generates a voltage proportional to the absolute temperature (which is referred to as a “PTAT voltage”), and a reference voltage generator for generating a reference voltage. The outputs from the PTAT voltage generator and the reference voltage generator are compared at a comparator, which is also included in the temperature sensor.
When the PTAT voltage exceeds the reference voltage (which is the target temperature at which the operation of the semiconductor integrated circuit has to be stopped), a chip enable (CE) signal for stopping the semiconductor integrated circuit is activated.
Both the PTAT voltage and the reference voltage have to be very precise, because if the precision of these voltages is degraded, the CE signal may be activated in spite of the fact that the semiconductor integrated circuit operates at an acceptable operating temperature, or the CE signal may not be activated even if the temperature exceeds the acceptable operative temperature. In the latter case, the semiconductor integrated circuit will be destroyed. Therefore, it is important for the temperature protection circuit to output the PTAT voltage and the reference voltage at high precision.
FIG. 1 illustrates a semiconductor temperature sensor disclosed in JPA H9-243466, which includes two MOS transistors N1 and N2 having different W/L ratios of the channel width W to the channel length L. If the same electric current Id is supplied from the constant voltage source 11, the gate-source voltages Vgs1 and Vgs2 generated at transistors N1 and N2 differ from each other. The potential difference between the two gate-source voltages (Vgs1−Vgs2) is in proportion to the operating temperatures of the transistors N1 and N2, and therefore, this voltage difference can be used as a PTAP signal. By adjusting the W/L ratios of the two MOS transistors N1 and N2, a signal having a positive or negative temperature coefficient can be obtained.
FIG. 2 illustrates another example of the temperature sensor, in which an NPN transistor or a PNP transistor is inserted by diode-connection. When a constant current is supplied, voltage Vt having a negative temperature coefficient appears between both ends of the diode. If the Vt exceeds the reference voltage Vref, a prescribed signal Tout is output from the comparator.
However, there is a problem in the semiconductor temperature sensor shown in FIG. 1 that outputs the potential difference between the gate-source voltages Vgs of two MOS transistors. The problem is that the temperature range for accurately extracting the PTAP voltage is limited to the range from −50° C. to 100° C. The accuracy of the PTAP voltage cannot be guaranteed at a higher temperature above 100° C., at which the semiconductor integrated circuit is very likely to be destroyed.
The circuit shown in FIG. 2 using a diode connection is also disadvantageous because the PTAP voltage is adversely affected by process variation, and is incapable of outputting a precise PTAT voltage. In addition, the slope of the PTAT voltage in proportion to the absolute temperature (i.e., the temperature slope) cannot be adjusted freely because the temperature slope is fixed by the process. The structure shown in FIG. 2 is lacking in flexibility for designing different types of circuits, such as a highly precise temperature sensing circuit with a large temperature slope, or a low-voltage operating circuit with a small temperature slope.
Another problem in the prior art techniques is difficulty in producing a constant and stable reference voltage, with which the detected PTAT voltage is compared, independently of the temperature.